We observe a twin plane re-entrant based growth mechanism for Au catalyzed Ge nanowire growth using video-rate lattice-resolved environmental transmission electron microscopy. For a [112] growth direction, we find a convex, V-shaped liquid catalyst-nanowire growth interface, composed of two Ge (111) planes that are truncated towards the triple phase boundary, that allows the interface to advance in discrete steps. The nanoscale geometry allows steady state growth based on a single twin boundary at the nanowire centre. We propose that the nucleation barrier at the twin plane re-entrant groove is effectively reduced by the line energy and hence the twin acts as a preferential nucleation site for atomic steps, which dictates the observed growth behavior.